This freaked me out somewhat. It's outside of my experience and I was really worried about it. Also, it felt like a single attack that happened in a flash, and it alarmed me how powerful Duke is suddenly.
In the light of the next morning - and having had an email chat with Fee, who has owned loads of lurchers,(and is also one of those people that has a real knack for asking exactly the right question at the right time to make you think) this is what I think happened:
Other dog was standing standing still, tail waving and relaxed stance. As it was a medium-larger dog and seemed confident, I didn't put Duke on the lead before we got to him, but decided to let him greet off lead.
Duke suddenly went into a sort of border collie style crouch, then in a split second, he launched himself like a missile at the other dog.
Az (the little toad) was running nearby at the same moment.
Az sheered off at once when I shouted, but to the other dog it must have seemed as though both dogs were going for him.
As Duke ran at him, the other dog's stance changed: he was clearly scared. At the point when Duke got to him, the other dog started to run and Duke ran past, then turned and went after him. At that point Duke looked like a hare coursing dog going after a hare.
He caught the other dog, knocked him over, and grabbed at his stomach. This happened behind where I was standing, so they must have run maybe10-20 yards and come past me before Duke knocked him over.
So, I THINK what Duke did was rush up far too fast and really in-your-face, all excited because he was joining in with Az - the other dog, seeing two dogs apparently attacking, understandably bolted, then Duke chased him down, getting frantically excited by the chase so when it ended he behaved as though he had caught a rabbit.
It seemed like all one action because it was so quick and at first my mind blurred the whole incident together.
This is the second such incident. The first occurred on Sunday, when I went for a group walk with some other sighthounds. An offlead collie ran right past three large sighthounds that were on lead. All three of the sighthounds grabbed for her (she was being very rude and provocative) and Duke managed to yank the lead right out of my hand and connected with her. I thought he was just joining in with the gang that time, but maybe it was a bit more than that.
Things learned and things to be done:
1) I need to be much more careful about where and when Duke goes offlead
2) Duke really needs better recall and I'd better start working on that seriously
3) Az and Duke never offlead at the same time unless there are really NO other dogs about at all (except Mollydog who is too old and sensible to get involved in all this lurcher laddish bad behaviour)
4) Duke must wear his muzzle to greet unfamiliar dogs.
5) Duke can't be trusted around dogs that may run away from him: he is too excitable (and getting very strong now!)
6) I have made an appointment for Duke to see a dog behaviour expert next week to get some help with recall training and to make sure that I'm not misreading this.
7) possibly I should get a no-pull style collar on Duke to stop him being able to pull the lead out of my hand. I'm thinking maybe the Canny Collar.